This invention relates to a d-c power supply, and more specifically relates to a novel circuit arrangement for a d-c power supply which produces low ripple d-c from a relatively low frequency a-c source, wherein the converter has an exceptionally high power factor with the use of relatively few and inexpensive components.
Numerous power supplies are known for supplying a relatively high level d-c voltage such as 300 V from a relatively low frequency a-c power line which might have a frequency of 50 Hz or 60 Hz. Such devices have almost universal application. One particular application of such a device is shown in co-pending application Ser. No. 966,604 filed Dec. 5, 1978, in which a high frequency converter is driven from the d-c output of a rectifier network which is, in turn, powered from the relatively low frequency of the a-c mains in a building or the like. Conversion of a-c power to low ripple d-c power for input to a high frequency converter can be obtained with many known circuits. However, a relatively inexpensive rectifier network is desired which can convert power to d-c at reasonable cost and in such a way that does not unduly load building wire or the power generating station that supplies the system.
Thus, the device preferably should be inexpensive in construction, easy to manufacture and reliable and should have a relatively high power factor.
The power factor of a rectifying network will be maximized when the phase difference between the line voltage and line current is minimal and further when the duty cycle of the line current is maximized. The duty cycle of the line current is the per unit time that current flows from the line during each half cycle of line voltage.
To understandd why a maximum duty cycle will maximize power factor, consider that the average power delivered from the line is: ##EQU1## where v is instantaneous voltage, i(t) is instantaneous current, t is time, V is rms voltage, and T is the period.
The effect of i(t) is, therefore, averaged over the cycle. However, the power factor is the ratio of the average power P to the volt ampere product V.times.I, where I is the rms current defined as: ##EQU2## The rms current I increases for a lower duty cycle because the waveform of i(t) is more peaked or has a shorter conduction period.
Many prior art rectifier circuits will be seen to have a poor power factor because their conduction time is short.